[w3c/push-api] Mention or enforce userVisibleOnly check in PushManager.subscribe? (#339)

After some testing, it seems that both Chrome and Firefox are disallowing calling PushManager.subscribe with https://w3c.github.io/push-api/#dom-pushsubscriptionoptions-uservisibleonly not present or set to false.
It would be good to indicate this possibility in the specification, for instance as a guideline, or directly within the subscribe algorithm directly.

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